ANALYZE

Name

ANALYZE -- collect statistics about a
database

Synopsis

ANALYZE [ VERBOSE ] [ table_name [ ( column_name [, ...] ) ] ]

Description

ANALYZE collects statistics about the
contents of tables in the database, and stores the results in the
pg_statistic system catalog. Subsequently,
the query planner uses these statistics to help determine the
most efficient execution plans for queries.

With no parameter, ANALYZE examines
every table in the current database. With a parameter, ANALYZE examines only that table. It is further
possible to give a list of column names, in which case only the
statistics for those columns are collected.

Parameters

VERBOSE

Enables display of progress messages.

table_name

The name (possibly schema-qualified) of a specific table
to analyze. If omitted, all regular tables (but not foreign
tables) in the current database are analyzed.

column_name

The name of a specific column to analyze. Defaults to
all columns.

Outputs

When VERBOSE is specified, ANALYZE emits progress messages to indicate which
table is currently being processed. Various statistics about the
tables are printed as well.

Notes

Foreign tables are analyzed only when explicitly selected. Not
all foreign data wrappers support ANALYZE. If the table's wrapper does not support
ANALYZE, the command prints a warning
and does nothing.

In the default PostgreSQL
configuration, the autovacuum daemon (see Section 23.1.6) takes
care of automatic analyzing of tables when they are first loaded
with data, and as they change throughout regular operation. When
autovacuum is disabled, it is a good idea to run ANALYZE periodically, or just after making major
changes in the contents of a table. Accurate statistics will help
the planner to choose the most appropriate query plan, and
thereby improve the speed of query processing. A common strategy
for read-mostly databases is to run VACUUM and ANALYZE
once a day during a low-usage time of day. (This will not be
sufficient if there is heavy update activity.)

ANALYZE requires only a read lock on
the target table, so it can run in parallel with other activity
on the table.

The statistics collected by ANALYZE
usually include a list of some of the most common values in each
column and a histogram showing the approximate data distribution
in each column. One or both of these can be omitted if ANALYZE deems them uninteresting (for example, in
a unique-key column, there are no common values) or if the column
data type does not support the appropriate operators. There is
more information about the statistics in Chapter 23.

For large tables, ANALYZE takes a
random sample of the table contents, rather than examining every
row. This allows even very large tables to be analyzed in a small
amount of time. Note, however, that the statistics are only
approximate, and will change slightly each time ANALYZE is run, even if the actual table contents
did not change. This might result in small changes in the
planner's estimated costs shown by EXPLAIN. In rare situations, this
non-determinism will cause the planner's choices of query plans
to change after ANALYZE is run. To avoid
this, raise the amount of statistics collected by ANALYZE, as described below.

The extent of analysis can be controlled by adjusting the
default_statistics_target
configuration variable, or on a column-by-column basis by setting
the per-column statistics target with ALTER
TABLE ... ALTER COLUMN ... SET STATISTICS (see ALTER TABLE). The target value sets the
maximum number of entries in the most-common-value list and the
maximum number of bins in the histogram. The default target value
is 100, but this can be adjusted up or down to trade off accuracy
of planner estimates against the time taken for ANALYZE and the amount of space occupied in
pg_statistic. In particular, setting the
statistics target to zero disables collection of statistics for
that column. It might be useful to do that for columns that are
never used as part of the WHERE,
GROUP BY, or ORDER
BY clauses of queries, since the planner will have no use
for statistics on such columns.

The largest statistics target among the columns being analyzed
determines the number of table rows sampled to prepare the
statistics. Increasing the target causes a proportional increase
in the time and space needed to do ANALYZE.

One of the values estimated by ANALYZE is the number of distinct values that
appear in each column. Because only a subset of the rows are
examined, this estimate can sometimes be quite inaccurate, even
with the largest possible statistics target. If this inaccuracy
leads to bad query plans, a more accurate value can be determined
manually and then installed with ALTER TABLE
... ALTER COLUMN ... SET (n_distinct = ...) (see ALTER TABLE).

If the table being analyzed has one or more children,
ANALYZE will gather statistics twice:
once on the rows of the parent table only, and a second time on
the rows of the parent table with all of its children. This
second set of statistics is needed when planning queries that
traverse the entire inheritance tree. The autovacuum daemon,
however, will only consider inserts or updates on the parent
table itself when deciding whether to trigger an automatic
analyze for that table. If that table is rarely inserted into or
updated, the inheritance statistics will not be up to date unless
you run ANALYZE manually.

If the table being analyzed is completely empty, ANALYZE will not record new statistics for that
table. Any existing statistics will be retained.